Incineration process using high oxygen concentrations

ABSTRACT

A process for incinerating combustible materials including the steps of: delivering combustible material and inlet gases to a primary combustion chamber, the inlet gases having an oxygen content of at least 50 vol %; burning the combustible material with the oxygen of the inlet gases in the primary combustion chamber producing flue gases and solid particulates as thermal decomposition products of the burnt combustible material; passing the flue gases and particulates to a secondary combustion chamber where further combustion occurs; cooling the flue gases exiting the secondary combustion chamber; returning a portion of the cooled flue gases to at least one of the combustion chambers where the cooled gases moderate the temperature in the at least one chamber; and passing the remaining portion of cooled flue gases on to a flue gas purification system where pollutants in the flue gases and particulates are substantially converted to benign compounds or removed entirely before the flue gases are emitted into the atmosphere.

The present application is a 35 USC 371 national phase application fromand claims priority to international application PCT/IL02/00503, filed24 Jun. 2002, established under PCT Article 21(2) in English, whichclaims priority to Israeli patent application Ser. No. 143993, filed 26Jun. 2001, which applications are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a method for incinerating combustiblematerials, particularly waste materials, including hazardous andbio-hazardous waste materials.

BACKGROUND OF THE INVENTION

The disposal of waste is a serious problem to governments, especiallymunicipal governments. The waste disposal process is regulated byincreasingly stricter standards since some wastes are toxic. In the caseof industrial waste, there are even more problematic materials, such aspetrochemicals, PCBs (polychlorinated biphenyls), etc. than in common,non-industrial waste. Additionally, medical and other biological wasteis often hazardous and requires complete sterilization anddecomposition.

Previously, other methods of waste disposal were more attractive thanincineration. Landfills, for example, were used instead of incinerationsince the cost of disposing waste at a landfill was far less than thatof incineration. However, increasingly more severe environmentalstandards have made landfills less attractive, primarily because of theincreased awareness that toxic chemicals, over long periods of time,percolate through the ground contaminating aquifers. Similarly, the everincreasing quantity of waste make landfills and other methods physicallyimpractical.

Accordingly, destructive, degradative processes such as incinerationhave become more popular. Destructive techniques like incineration mustefficiently turn waste into innocuous end-products. This is aparticularly acute problem in incineration where burning hazardous wasterequires high temperatures so that the resulting decomposition productsare environmentally benign. The high temperatures needed and the largequantities of waste involved require the development of incineratorsthat are economically and environmentally efficient. The emissions fromsuch products are generally gaseous and must comply with standards setby international and governmental agencies. Similarly, solid andparticulate wastes of incineration, such as slag, bottom ash and flyash, must be neutered to remove harmful effects to the environment.

Examples of recently proposed incineration methods and incinerators canbe found in U.S. Pat. Nos. 5,752,452 and 5,179,903, and WO 96/24804,Abboud. U.S. Pat. No. 5,179,903 and WO 96/24804 describe recycled fluegases which are augmented with oxygen, U.S. Pat. No. 5,752,452 describesa system with lances which inject oxygen into a heating zone at avelocity of at least 350 ft/sec.

However, despite improvements in incinerators and incinerationprocesses, capital and maintenance costs are still very high. Inaddition, effluents emitted into the environment still require furtherreduction.

SUMMARY OF THE PRESENT INVENTION

An object of the present invention is to provide a process whichmaximizes the rate of incineration and throughput in waste incineratorswhile minimizing gas emissions and solid waste produced.

It is a further object of the present invention to provide an economicalincineration process for use with industrial, consumer and biologicalwastes, including hazardous waste.

It is yet another object of the present invention to minimize the sizeof the required incinerator and flue gas purification system, therebyminimizing the required investment and maintenance costs.

A further object of the present invention is to provide an economical,environmentally friendly process which can be applied to largeindustrial installations, such as electricity generating plants, whichburn large quantities of fossil fuels.

There is thus provided in accordance with the present invention aprocess for incinerating combustible material including the step ofdelivering combustible material and inlet gases to a primary combustionchamber, the inlet gases having an oxygen content of at least 50 vol. %.This is followed by burning the combustible material with the oxygen ofthe inlet gases in a primary combustion chamber producing flue gases andsolid particulates as thermal decomposition products of the burntcombustible material. The flue gases and particulates are then passed toa secondary combustion chamber where further combustion occurs. The fluegases exiting from the secondary combustion chamber are cooled. Aportion of the cooled flue gases is returned to at least one of thecombustion chambers where the cooled gases moderate the temperature inthe at least one chamber. Finally, the remaining portion of the cooledflue gases is passed on to a flue gas purification system wherepollutants in the flue gases and particulates are substantiallyconverted to benign compounds or removed entirely before the flue gasesare emitted into the atmosphere.

Additionally, there is provided in accordance with the present inventiona process which further includes the step of monitoring the value of atleast one parameter in at least one combustion chamber, the parameterbeing a function of the thermal decomposition of the combustiblematerial in at least one combustion chamber. This is followed bycomparing the value of the at least one monitored parameter with atleast one predetermined value for that parameter, the comparison beingeffected by a control device. Finally, the result of the comparison iscommunicated to a means for controlling the portions of cooled fluegases returned to the at least one combustion chamber and the flue gaspurification system. The means for controlling the portions adjusts therelative sizes of the two portions accordingly.

Additionally, in accordance with a preferred embodiment of the presentinvention the at least one parameter in the monitoring step istemperature. The temperature can be monitored in the primary combustionchamber or in the secondary combustion chamber or in both chambers.

Further, in accordance with a preferred embodiment of the presentinvention, the at least one parameter in the monitoring step is theconcentration of carbon monoxide or the concentration of oxygen or theconcentration of both simultaneously. These concentrations can bemeasured in the effluent of the secondary combustion chamber.

In accordance with a preferred embodiment of the present invention, themeans for controlling the amount of cooled gases are valves.

Additionally, in accordance with a preferred embodiment of the presentinvention, the inlet gases of the delivering step are delivered in twohigh concentration oxygen streams, one inlet gas stream positionedadjacent to the burning waste and the other above the flames of theburning waste, the amount of oxygen from each stream controlled so thatthe temperature of the burning waste is maintained at a temperature thatdoes minimal damage to the floor of the primary combustion chamber,while ensuring complete combustion of the waste and an oxygen volume %in the system's effluent within regulatory limits.

Further, in accordance with a preferred embodiment of the presentinvention the oxygen content of the inlet gases is at least 80 vol. %.

Additionally, in a preferred embodiment of the present invention, theoxygen content of the inlet gases is at least 90 vol. %.

Further, in a preferred embodiment of the present invention, the oxygencontent of the inlet gases is between about 90 vol. % and 95 vol. %.

Additionally, in accordance with a preferred embodiment of the presentinvention, the burning step in the primary combustion chamber iseffected at a temperature from about 1100° C. to about 2000° C.

In another preferred embodiment of the present invention, the burningstep in the primary combustion chamber is effected at a temperature fromabout 1200° C. to about 1750° C.

Additionally, in a preferred embodiment of the present invention, theburning step in the primary combustion chamber is effected at atemperature from about 1300° C. to about 1500° C.

Further, in yet another embodiment of the present invention, combustionin the secondary combustion chamber of the first passing step iseffected at a temperature from about 850° C. to about 1500° C.

In another embodiment of the present invention, combustion in thesecondary combustion chamber of the first passing step is effected at atemperature from about 950° C. to about 1350° C.

Additionally, in yet another embodiment of the present invention,combustion in the secondary combustion chamber of the first passing stepis effected at a temperature from about 1050° C. to about 1200° C.

In another embodiment of the present invention, the process furtherincludes the step of adding at least one reduced nitrogen compound intothe second combustion chamber to destroy nitrogen oxide gases.Typically, the at least one reduced nitrogen compound can be ammonia orurea.

Further, in a preferred embodiment of the present invention, the processfurther includes the step of separating solid particulates from the fluegases after the gases are cooled.

Additionally, in a preferred embodiment of the invention, the at leastone combustion chamber of the returning step is the primary combustionchamber.

Finally, in a preferred embodiment of the present invention, the cooledflue gases are returned to the primary combustion chamber proximate tothe flame produced by burning combustible material in that chamber. Inanother embodiment, the cooled flue gases are returned to the primarycombustion chamber proximate to the bottom ash and slag.

In yet another preferred embodiment of the present invention, the atleast one combustion chamber of the returning step is the secondarycombustion chamber.

Finally, the present invention can be used with combustible materialwhich is waste, including hazardous waste, or fuels.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a flow diagram illustrating a preferred embodiment of theprocess of the present invention;

FIG. 2A is a schematic view of an incinerator operative in accordancewith the present invention;

FIG. 2B is a schematic view of a typical purification system which canbe used with an incinerator operative in accordance with the presentinvention; and

FIG. 3 is a schematic diagram illustrating another preferred embodimentof the process of the present invention.

Similar elements in the Figures are numbered with similar referencenumerals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1, which shows a flow diagram of apreferred embodiment of an incineration process generally referenced110, in accordance with the present invention. Process 110 isparticularly preferred when used to incinerate industrial, commercialand/or biological waste. The description herein below, as well as theaccompanying Figures, describe the process in terms of such waste.However, while the above process 110 has been discussed as a process forthe incineration of waste, the system can also be used to burn any fuel,producing energy in a clean, cost efficient manner. In lieu of municipalor industrial waste, process 110 can be used to burn fuels such asnatural gas, fuel oil, and coal. These fuels, however, are to be viewedas non-limiting examples.

Process 110 includes a primary combustion chamber (PCC) 12 into whichwaste is fed 51 via a conduit (not shown). Inlet gases containing atleast 50 vol. %, preferably at least 80 vol. %, and most preferably atleast 90 vol. % oxygen, usually between about 90 vol. % to 95 vol. %oxygen, are also passed 53, via a conduit (not shown), into PCC 12,typically in the region immediately proximate to the burning waste. Thewaste is burned in an excess of the stoichiometric amount of oxygen. Thewaste is burned in PCC 12 at temperatures maintained between about 1100to 2000° C., preferably between about 1200 to 1750° C., and even morepreferably between about 1300 to 1500° C. Because of the high oxygenconcentrations used in primary combustion chamber 12, a significantpercentage of the material burned undergoes complete oxidation. Oxygenlancing and other methods to introduce supplementary oxygen aretherefore not required.

Flue gases mixed with small solid particulates resulting fromincineration rise from PCC 12 and pass 55, via a conduit (not shown),into a secondary combustion chamber (SCC) 14. Partially combusted fluegases are further combusted in SCC 14 to more completely oxidized gasesusing the residual oxygen arriving from PCC 12. In SCC 14, thetemperature is maintained within the range of from about 850 to 1500°C., preferably from about 950 to 1350° C., and even more preferably fromabout 1000 to 1200° C.

Optionally, materials which destroy nitrogen oxide gases (NOx) are fed57, via a conduit (not shown), into SCC 14. Typically, these materialsare reduced nitrogen compounds such as ammonia or urea which convert theNOx gases formed in PCC 12 and SCC 14 into nitrogen and water. Since theamount of nitrogen comprising the inlet gases passed 53, via a conduit(not shown), into PCC 12 is small, the amount of NOx present in thesystem is not great. In some embodiments, the materials which destroynitrogen oxide gases may be employed without a catalyst; in otherembodiments, a catalyst may be required. Preferably, PCC 12 and SCC 14are contained in a single structure, but each can be located in separatestructures, when necessary.

The flue gases are conveyed 59 via a conduit (not shown) to a heatexchanger 22. Typically, heat exchanger 22 may be a boiler which removesheat from the flue gases. The energy removed, usually as steam, isconveyed 61, via a conduit (not shown), to an energy converter 18, oftena turbogenerator. Alternatively, any heat recovery system from whichelectricity or steam can be withdrawn 63 can be employed. Anyelectricity generated or steam removed can be returned to theincineration plant or distributed to outside consumers.

After emerging from heat exchanger 22, the flue gases have a temperatureof about 230 to 270° C., preferably about 250° C. The gases aretransferred 65 via a conduit (not shown) to a particulate separator 26,typically a cyclone separator, which via a conduit (not shown), removes69 fly ash 27 from the flue gases. The removed fly ash 27 is collected,“bagged” and sent to a toxic waste disposal site. The use of aparticulate separator 26 at this stage of process 110 is optional.Alternatively, particulates can be removed exclusively in flue gaspurification system 29 discussed below. As another alternative,purification system 29 can include a particulate remover whichsupplements particulate separator 26.

Two valves (not shown) located between particulate separator 26 and fluegas purification system 29 divide the flue gases into two portions. Thepercentage of flue gas that is recycled 73 through a conduit (not shown)and the percentage of flue gas passed 71 via a conduit (not shown)directly on to a flue gas purification system 29 for furtherpurification is determined by some parameter(s) of PCC 12 and/or SCC 14.Typically, the parameter is its (their) temperature(s) or theconcentration of carbon monoxide and/or oxygen on the downstream side ofSCC 14.

The flue gases that are passed on 71 via a conduit (not shown) forfurther purification reach flue gas purification system 29, details ofwhich are not shown. The exact nature of purification system 29 dependson the waste being incinerated, the gases and particulates emitted, andthe environmental standards which must be met. Typically, flue gaspurification system 29 contains a particulate remover, which supplementsoptional particulate separator 26, discussed above, and sometimes servesas the sole particulate remover in process 110. Generally, theparticulate remover in purification system 29 traps finer particles thanoptional particulate separator 26. Typically, purification system 29also contains a scrubber to neutralize acid gases. Other apparatusescommonly used for purifying effluent gases can be added as needed toattain the required effluent emission standards before the gases areexpelled 81 to the atmosphere.

Another portion of the flue gases is recycled 73 via a conduit (notshown) to PCC 12. Typically, the recycled, cooled flue gases arereturned 73A via a conduit (not shown) to PCC 12 directly above theflame, thereby removing heat from PCC 12 and transferring it to heatexchanger 22 via SCC 14. In another embodiment, the recycled flue gasescan be returned 73B via a conduit (not shown) directly to SCC 14. In yetanother embodiment, the flue gases can also be recycled 73C via aconduit (not shown) through bottom ash and slag 17 lying at the floor ofPCC 12. Finally, in other embodiments, the cooled flue gases can bereturned to both PCC 12 and SCC 14. Because PCC 12 operates attemperatures in excess of 1300° C., the bottom ash becomes vitrified 75when cooled. Some ash is carried 79 by convection to SCC 14. Cooled slagand vitrified bottom ash 17 are periodically removed 77 to a slag andbottom ash receptacle (not shown) for disposal.

Reference is now made to FIG. 2A which shows a schematic view of anincinerator system 210 operated in accordance with the process 110 ofthe present invention shown in FIG. 1. The system 210 permits a betterunderstanding of process 110 presented in FIG. 1. The system shown inFIG. 2A, however, is presented by way of example only and should not beconsidered as limiting.

System 210 includes a primary combustion chamber 12 into which waste 19is fed from a waste feed 10. There is an inlet gas feed array 15 whichdelivers inlet gases for combustion, the gases typically being composedof at least 90 vol. % oxygen. Waste 19 is burned in primary combustionchamber (PCC) 12. The inlet gases are brought from array 15 proximate tothe burning waste in PCC 12. The high concentration of oxygen in theinlet gases fed to primary combustion chamber 12 accelerates the rate ofcombustion of waste 19. The temperature in PCC 12 is also significantlyhigher than temperatures generated when air alone is used. The highertemperatures attained easily crack and shatter solids, facilitatingtheir incineration. Materials that do not burn in air, or do so onlyincompletely, burn easily in inlet gases with a high oxygen content,often to near completion. Since the oxygen concentration used in theprocess of the present invention is so high, burning is much morecomplete and there is no need for selectively introducing lanced oxygen.Because the rate of combustion is faster than in currently usedincinerators, primary combustion chamber 12 can be made smaller whilethroughput will be greater than in prior art incinerators.

PCC 12 has a bottom grating comprised of slats, which are preferablyadapted to be rotatable or otherwise movable so as to rotate orotherwise agitate the burning waste. The grating can be made from, orcovered with, ceramic materials which protect it from the elevatedtemperature of combustion. Typically, every other grating slat is movedperiodically, turning over the burning waste, permitting more thoroughand rapid combustion. The lower parts of the walls of PCC 12 must alsobe protected from the heat, usually using ceramic tiling as shields.Alternatively, the walls and the grating can be cooled with waterflowing through adjacent water pipes. It is readily apparent to oneskilled in the art that instead of grating slats at the bottom ofprimary combustion chamber 12, the floor of chamber 12 can includerotating metal cylindrical rollers or any other means that canperiodically move and/or rotate the burning waste.

Slag and bottom ash 17 from PCC 12 are cooled and emptied into an ashand slag receptacle (not shown) via a slag channel 16. Because of thehigh temperatures (>1300° C.) in the primary combustion chamber 12, thebottom ash 17 is vitrified when cooled and encapsulated in a glass-likecrust. The encapsulation insulates and neutralizes harmful materialsmaking them usable for civil engineering projects such as road bedswithout the need for further processing.

Gases and fly ash emitted from the burning waste as well as residualoxygen from PCC 12 enter a secondary combustion chamber 14 whereadditional combustion occurs. An array 30 of nozzles in the wall ofprimary combustion chamber 12 injects cooled, recycled flue gases intoPCC 12; typically these recycled gases enter PCC 12 immediately aboveflames 11. The cooled, recycled flue gases entering from array 30 have atypical temperature of approximately 250° C. and they maintain thetemperature in primary combustion chamber 12 at a predeterminedtemperature, generally about 1300 to 1500° C. Similarly, they cool thegases rising from PCC 12 into SCC 14 to temperatures between about 1000to 1300° C.

Optionally, ammonia or urea are added to the flue gas in SCC 14 reducingthe nitrogen oxide gases produced in PCC 12 and SCC 14 to nitrogen andwater. PCC 12 and SCC 14 can be constructed as any one of several typesof chambers, such as rotary kiln, fixed hearth or other types of ovens.

The gases continue on from secondary combustion chamber 14 to an heatexchanger 22, typically a boiler. Heat exchanger 22 removes heat fromthe flue gases, generally forming steam which is led to a turbogenerator(not shown). The turbogenerator can be connected to an electric gridfrom which electricity can be delivered directly to consumers orreturned to the incineration plant for use within the plant.Alternatively, the steam itself, or a mixture of steam and electricitygenerated by the heat exchanger/boiler 22 and turbogenerator (not shown)respectively, can be sold. By the time the gases and fly ash emissionsfrom the burnt waste reach an optional blower 24, the temperature of thegases has been reduced to approximately 250° C.

The fly ash that passes through optional blower 24 enters an optionalcyclone separator 26 which precipitates the bulk of the fly ash passingthrough blower 24. The cyclone separator 26 may be any cyclone separatorcommercially available used to separate particulates from gases. Asingle cyclone or multiple cyclones can be used.

It should be noted that there is a significant reduction in the amountof fly ash produced by the process of the present invention. Thereduction in fly ash is a direct consequence of the very high percentageof oxygen introduced with the inlet gases. The high percentage of oxygenreduces the total amount of inlet gases provided to primary combustionchamber 12, which in turn leads to a smaller volume of carrier gas forash generated by incineration. More of the ash produced remains asbottom ash. Since fly ash traps poisonous materials found in flue gases,such as dioxins and heavy metals, the law requires that fly ash begathered and delivered to a toxic disposal dump. Any reduction in flyash therefore results in a reduction in waste treatment expense.

The bulk of the emitted waste gases, the flue gas, is returned via arecycling line 28 to primary combustion chamber 12. The recycled fluegas is at a temperature of approximately 250° C. and enters PCC 12through array 30 in the wall of primary combustion chamber 12.Generally, the gases enter the chamber proximate to and above flames 11.The cooled recycled flue gas functions as a coolant keeping thetemperature in primary combustion chamber 12 at the predeterminedtemperature, typically 1300-1500° C. Typically, the recycled flue gasesreenter the system directly into PCC 12 above flames 11 therein;optionally they can also be recycled directly to SCC 14 or into thebottom ash and slag 17 on the floor of PCC 12. Typically, an array ofconduits is used for reintroducing the recycled flue gas, but in otherembodiments, a single point of entry for the recycled flue gases may beemployed.

Part of the flue gases from blower 24 enters a cleaning line 32. Valves31A and 31B determine how much, and when, flue gases enter cleaning line32 and recycling line 28. Using 90 vol. % oxygen and a typical mix ofIsraeli municipal waste, the mixture of flue gases generated andentering these lines has a typical approximate composition of oxygen 6vol. %, nitrogen 5 vol. %, CO₂ 43 vol. % and steam 46 vol. %. If theinlet gases fed to primary combustion chamber 12 had been air(approximately 21 vol. % oxygen) and not a gas mixture containing atleast 90 vol. % oxygen, the nitrogen content of the flue gases enteringcleaning line 32 and recycling line 28 would have risen to approximately66 vol. %.

Valves 31A and 31B are connected to a control system which monitors aparameter, typically the temperature, of the gases exiting primarycombustion chamber 12 and/or secondary combustion chamber 14. If thetemperature is higher than required, a larger percentage of the fluegases is recirculated to the primary combustion chamber; if thetemperature in the primary combustion chamber is lower than required,the amount of flue gases that is returned is decreased. If, for example,the temperature in PCC 12 is 1750° C. and the temperature in SCC 14 is1100° C., the approximate percentage of flue gases recycled is 60 vol. %while 40 vol. % are passed via line 32 directly to the flue gaspurification system 310 shown in FIG. 2B and discussed below.

Typically, a device, for example a thermocouple, is used to measure thetemperature inside PCC 12 and/or SCC 14, while a temperature controllercompares the measured PCC 12 and SCC 14 temperatures, with one or moretemperature set points. The controller then opens or closes the twovalves accordingly, returning the required amount of recycled flue gasesto PCC 12 and/or SCC 14. The recycling of cooled flue gases ensuresbetter control of temperature in primary combustion chamber 12 than whenrecycling is absent. It also increases the degree of combustion of theflue gases.

Reference is now made to FIG. 2B, where a schematic view of an exemplarypurification and scrubbing system 310 of the incinerator plant is shown.The configuration of devices in FIG. 2B are shown merely by way ofexample and the scope of the present invention is not intended to belimited thereby.

Cleaning line 32 continues into the purification system 310 of the plantwhere the amount of effluent solid and flue gases is reduced. Thesegases and solids are led into an electrostatic precipitator (ESP) 34which complements or functions in place of cyclone separator 26discussed above. In ESP 34 much of the remaining fly ash is removed. InESP 34, fly ash particulates are charged by a high voltage source anddrawn to a conductive plate of opposite charge where the particulate'scharge is dissipated. The ash is then precipitated and collected.

The flue gases are then sent via a line 42 to a scrubber heat exchanger36 which removes heat from the system. The gases enter the lower part ofa scrubber 40 where the temperature is less than 100° C. and much of thewater vapor in the flue gases condenses. In scrubber 40, drops of abasic solution containing calcium hydroxide, sodium hydroxide, sodiumcarbonate, potassium carbonate or some other such alkaline compound areinjected. These neutralize acid gases such as sulfur dioxide and anyresidual nitrogen oxides not destroyed by ammonia or urea optionallyadded in secondary combustion chamber 14. The scrubbed gases thenreenter heat exchanger 36, via a line 38, where they are reheated usingthe heat previously withdrawn from the flue gases before these gasesentered the lower part of scrubber 40. The reheated gases then enter aline 46, where an activated carbon injector 44 injects carbon into line46, so that contaminants, among them dioxins and furans, are adsorbed.The carbon also traps other contaminants including heavy metal and heavymetal oxide particulates.

The injected activated carbon and gas effluents advance through line 46and are deposited onto a fabric filter 50, which removes the injectedactive carbon from the flue gases. Residual gases such as oxygen andnitrogen are then led through a line 48 to a stack 52 where they areemitted into the air, usually with the assistance of a blower 49 locatedat the bottom of the stack.

When the inlet gases contain at least 90% oxygen, the amount of effluentgases emitted from stack 52 is about 5 times less than the amountemitted by currently used incinerators. Typically, approximatepercentages of the emitted gases using the process of the presentinvention are 6 vol. % oxygen, 5 vol. % nitrogen, 20 vol. % water vaporand 70 vol. % carbon dioxide.

The reduction in nitrogen and the large amount of completely oxidizedcarbon in the form of carbon dioxide are a direct result of the use ofinlet gases with a very high oxygen content followed by recycling offlue gases into the primary combustion chamber. The reduction in watervapor is a consequence of the condensation of a large percentage of thevapor in scrubber 40 discussed above.

It should be apparent to one skilled in the art that the exactconfiguration of devices used to clean the effluent after it enterscleaning line 32 is to a degree variable and/or optional. Other types ofscrubbers and filters known in the art can be used. Similarly, some ofthe devices discussed above may be absent entirely while others notshown can be added. Cleaning devices at different plants would beexpected to vary depending on the nature of the waste being burned andthe environmental standards which must be met.

The inlet gases used to burn waste in primary combustion chamber 12 ofthe process discussed herein above should typically contain at least 80vol. %, preferably at least 90 vol. %, but generally between 90 vol. %and 95 vol. %, oxygen. This level of oxygen content (90-95 vol. %) isreadily attained by using a vapor pressure swing adsorption (VPSA)device, such as the one produced by Praxair Inc. A VPSA device absorbsnitrogen from air and passes the rest of the gases, mainly oxygen, toprimary combustion chamber 12 at relatively low cost. VPSA separatesnitrogen from air by molecular sieving. Nitrogen is adsorbed at lowpressures in the sieve and then removed by vacuum. Presently, thismethod is the most economical way to obtain gas fractions having suchhigh percentages of oxygen. Any attempt to use higher concentrations ofoxygen to increase the performance of the incinerator would increase thecost of producing the inlet gas because it would require distillation ofliquefied air.

The use of VPSA as discussed above or, alternatively, the relatedpressure swing adsorption (PSA) process to produce inlet gasescontaining a high percentage of oxygen should be viewed as non-limiting.Devices employing membrane technology also can be used to produce inletgases with higher than atmospheric oxygen content but these typicallyare only 40 to 60 vol. %.

Since there is likely to be a reduction by a factor of about 5 ineffluent gases at the incinerator's stack when the inlet gases of theincinerator include at least 90% oxygen (based on absolute amount ofweight per ton of waste), there is a concomitant reduction in the sizeand cost of the apparatus required to clean up effluent gases.Similarly, costs of the incinerator are reduced because of the fastercombustion and higher throughput. In addition, because of the reductionin fly ash and the vitrification of bottom ash in the system, wastedisposal costs are reduced. Finally, because nitrogen forms a muchsmaller portion of the inlet gases, energy lost in heating nitrogen isreduced. This energy may be retrieved for profitable use elsewhere.

Reference is now made to FIG. 3 which schematically shows anotherembodiment of the present invention. FIG. 3 includes a primarycombustion chamber (PCC) 12, a secondary combustion chamber (SCC) 14,and their control systems 120, 122 and 124. It also includes an inletgas feed array 15, an auxiliary inlet gas feed array 115 and a recycledgas flue array 30, positioned in the aforementioned chambers.

In this, as in previous embodiments, a high oxygen concentration is fedinto PCC 12 proximate to the burning waste at the floor of PCC 12.Oxygen is delivered through inlet gas feed array 15, which, because ofthe high concentration of oxygen delivered, generates very hightemperatures near the burning waste 11. These temperatures may adverselyeffect the structure of PCC 12 and can require different, more heatresistant, more costly materials from which to construct PCC 12.

In order to reduce combustion temperatures in the bottom region of PCC12, the present embodiment contemplates limiting the total amount ofoxygen supplied to the primary chamber by inlet gas feed array 15.Limiting the oxygen introduced by array 15, but not the highconcentration of the oxygen, reduces the temperature at, or near, thefloor of PCC 12.

With the reduction in total amount of oxygen introduced through inletgas feed array 15, some waste, and the flue gases generated therefrom,may be incompletely oxidized. In order to ensure that all the waste andflue gases are substantially completely burned, there is positioned inPCC 12 a second gas feed array carrying a high concentration of oxygento PCC 12. This second array, herein denoted as an auxiliary inlet gasfeed array 115, supplies a high concentration of oxygen, typically inexcess of 90%, over the burning coals and into the flue gases risingtherefrom. The oxygen fed through auxiliary inlet gas feed array 115produces substantially complete combustion of the flue gases generatedby the burning waste in PCC 12, while permitting operation of PCC 12 atlower temperatures. Even if oxygen provided by auxiliary inlet gas feedarray 115 increases the temperature of the exiting flue gases, littleincrease in temperature results in the burning waste adjacent the floorof PCC 12 and little damage to the floor of PCC 12 occurs.

The temperature of the exiting flue gases is moderated by recycled gasesintroduced from an array of nozzles 30 through valves 130, the nozzlesgenerally located in the wall of SCC 14 or in the upper region of PCC12. The temperature of the exiting flue gases is measured by athermocouple, pyrometer or other temperature monitoring instrument 142Bconnected to a temperature control unit 120 which controls the operationof valves 130.

Using two high concentration oxygen sources, inlet gas feed array 15 andauxiliary inlet gas feed array 115, allows for substantially completecombustion of the waste at generally lower temperatures in, or proximateto, the burning waste located at, or near, the bottom of PCC 12.

The amounts of oxygen brought into PCC 12 and needed to maintainrelatively low combustion temperatures there can be controlled inseveral ways. Temperature control can be effected by monitoring theoxygen concentration in the effluent emerging from the system's stack52. As described above, flue gas concentrations entering the atmospheremust meet strict regulatory requirements. An oxygen monitoringinstrument 132 can be inserted into, or positioned near, the outlet ofstack 52 to monitor the oxygen vol. % of the effluent. Data relating tothe concentrations thus measured are then fed to an oxygen concentrationcontrol unit 122. When the oxygen concentration in the effluent emergingfrom stack 52 is lower than required by regulations, the amount ofoxygen provided by auxiliary oxygen feed array 115 is increased; whenthe amount of oxygen is higher than required by regulations, the amountof oxygen supplied by auxiliary oxygen feed array 115 is reduced.

As an alternative to an oxygen monitoring instrument 132 positioned atthe outlet of stack 52, oxygen can be monitored by measuring oxygencontent of the recycled gases delivered by recycled gas flue array 30and entering either PCC 12 or SCC 14. The percentage oxygen content atstack 52 is related to the oxygen content in the recycled gases arrivingfrom recycled gas flue array 30. Therefore, the composition of therecycled gases entering either PCC 12 or SCC 14 can be used to determinethe over or under abundance of oxygen at stack 52.

In yet other embodiments of the present invention, two oxygen monitoringinstruments can be used to determine the oxygen content exiting stack52. One instrument 132 can be positioned at stack 52 while the other canbe located at the point where recycled flue gases are delivered by array30.

An alternative method for controlling the system is by monitoring thetemperature in PCC 12. At least one thermocouple or pyrometer 142A isplaced near, or at, the flames 11 of the burning waste. The results ofthese temperature measurements then are fed into a control unit 124, theburning waste temperature control unit, and compared to a predeterminedtemperature setting. The amount of oxygen provided to PCC 12 by both gasinlet arrays 15 and 115 then is adjusted to maintain a predeterminedtemperature setting at flames 11 by operating valves 126 and 128,respectively. By controlling temperature, the effluent oxygenconcentration at stack 52 is also kept within regulatory limits.

It should be readily apparent to one skilled in the art that there is areciprocal relationship between the amount of oxygen being suppliedthrough valves 126 and 128 of inlet gas feed array 15 and auxiliaryinlet gas feed array 115, respectively. When more oxygen is required atarray 15, generally less oxygen is required at array 115 for a givenrequired flame temperature.

When the temperature of the burning material is too high, valve 126,controlled by burning waste temperature control unit 124, reduces theflow of oxygen from inlet gas feed array 15 above the burning coals.Control unit 124 is separate from another control unit, the temperaturecontrol unit 120, which monitors temperature at the exit of thesecondary combustion chamber (SCC) 14. This temperature, as discussedabove, is effected by means of two valves 31A and 31B (FIG. 2A) whichdetermine the amount of recycled cooled flue gases returned to PCC 12and SCC 14 or sent to stack 52 by recycling line 28 (FIG. 2A) orcleaning line 32 (FIG. 2A), respectively.

It can readily be seen that the temperature of the burning coals asmeasured by measuring instrument 142A and controlled by control unit 124through valve 126 and gas feed array 15, the oxygen monitoringinstrument 132 at stack 52 and its oxygen control unit 122 through valve128 and auxiliary gas feed array 115, and temperature monitoringinstrument 142B through temperature control unit 120 and valve 130 ofrecycled flue gas array 30 form three control loops which arefunctionally interconnected. Generally, changes in one have adiscernible effect in the other two control loops.

The embodiment shown in FIG. 3 moderates and controls temperature betterthan in currently available furnaces. This embodiment with its auxiliaryoxygen feed array 115 and recycled flue gas array 30, the latterpositioned either in the walls of secondary combustion chamber (SCC) 14or the upper region of PCC 12, permits moderation of the temperature atevery stage of the combustion process. Furnace temperatures,irrespective of the type of the furnace used, can be maintained so thatdamage to PCC 12 is minimized.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed herein above. Rather the scope of the invention is definedsolely by the claims that follow.

1. A process for incinerating combustible material including the stepsof: delivering combustible material and inlet gases to a primarycombustion chamber, the inlet gases having an oxygen content of at least50 vol. %; burning the combustible material with the oxygen of the inletgases in the primary combustion chamber producing flue gases and solidparticulates as thermal decomposition products of the burnt combustiblematerial; passing the flue gases and particulates to a secondarycombustion chamber where further combustion occurs; cooling the fluegases exiting the secondary combustion chamber; returning a portion ofthe cooled flue gases to at least one of the combustion chambers wherethe cooled gases moderate the temperature in the at least one chamber;passing the remaining portion of cooled flue gases on to a flue gaspurification system where pollutants in the flue gases and particulatesare substantially converted to benign compounds or removed entirelybefore the flue gases are emitted into the atmosphere; monitoring thevalue of at least one parameter of at least one combustion chamber, theparameter being a function of thermal decomposition of the combustiblematerial in at least one combustion chamber; comparing the value of theat least one monitored parameter with at least one predetermined value,the comparison being effected by a control device; and communicating theresult of the comparison to a means for controlling the portion ofcooled flue gases returned to the at least one combustion chamber insaid step of returning and to the flue gas purification system in saidstep of passing the remaining portion of cooled flue gases, the meanscontrolling the size of the portions accordingly.
 2. A process accordingto claim 1, where the at least one parameter in said step of monitoringis temperature.
 3. A process according to claim 1, where the at leastone combustion chamber is the secondary combustion chamber.
 4. A processaccording to claim 1, where the at least one combustion chamber is theprimary combustion chamber.
 5. A process according to claim 1, where theat least one parameter in said step of monitoring is the concentrationof carbon monoxide.
 6. A process according to claim 5, where the atleast one combustion chamber is the secondary combustion chamber.
 7. Aprocess according to claim 1, where the at least one parameter in saidstep of monitoring is the concentration of oxygen.
 8. A processaccording to claim 7, where the at least one combustion chamber is thesecondary combustion chamber.
 9. A process according to claim 1, whereinthe means for controlling the amount of cooled gases are valves.
 10. Aprocess according to claim 1, where the oxygen content of the inletgases is at least about 80 vol. %.
 11. A process according to claim 1,where the oxygen content of the inlet gases is at least about 90 vol. %.12. A process according to claim 1, where the oxygen content of theinlet gases is between about 90 vol. % and 95 vol. %.
 13. A processaccording to claim 1, where said step of burning in the primarycombustion chamber is effected at a temperature from about 1100° C. toabout 2000° C.
 14. A process according to claim 1, where said step ofburning in the primary combustion chamber is effected at a temperaturefrom about 1200° C. to about 1750° C.
 15. A process according to claim1, where said step of burning in the primary combustion chamber iseffected at a temperature from about 1300° C. to about 1500° C.
 16. Aprocess according to claim 1, further comprising the step of adding atleast one reduced nitrogen compound into the second combustion chamberto destroy nitrogen oxide gases.
 17. A process according to claim 16,wherein the at least one reduced nitrogen compound is ammonia or urea.18. A process according to claim 1, further comprising the step ofseparating solid particulates from the flue gases after the gases arecooled.
 19. A process according to claim 1, wherein the at least onecombustion chamber of said step of returning is the primary combustionchamber.
 20. A process according to claim 19, wherein said step ofburning of combustible material produces flames and the cooled fluegases are returned to the primary combustion chamber above the flames inthat chamber.
 21. A process according to claim 1, where the combustiblematerial is waste.
 22. A process according to claim 21, where thecombustible material is hazardous waste.
 23. A process according toclaim 1, where the combustible material is a fuel.
 24. A processaccording to claim 1, where the oxygen content of the inlet gases isbetween about 95 vol. % and about 100 vol. %.
 25. A process forincinerating combustible material including the steps of: deliveringcombustible material and inlet gases to a primary combustion chamber,the inlet gases having an oxygen content of at least 50 vol. %; burningthe combustible material with the oxygen of the inlet gases in theprimary combustion chamber producing flue gases and solid particulatesas thermal decomposition products of the burnt combustible material;passing the flue gases and particulates to a secondary combustionchamber where further combustion occurs; cooling the flue gases exitingthe secondary combustion chamber; returning a portion of the cooled fluegases to at least one of the combustion chambers where the cooled gasesmoderate the temperature in the at least one chamber; and passing theremaining portion of cooled flue gases on to a flue gas purificationsystem where pollutants in the flue gases and particulates aresubstantially converted to benign compounds or removed entirely beforethe flue gases are emitted into the atmosphere, wherein the inlet gasesof said step of delivering are delivered in two high concentrationoxygen streams, one inlet gas stream positioned adjacent to the burningcombustible material and the other above the flames of the burningcombustible material, the amount of oxygen from each stream controlledso that the temperature of the burning combustible material ismaintained at a temperature that does minimal damage to the floor of theprimary combustion chamber, while ensuring substantially completecombustion of the combustible material and an oxygen volume % in thesystem's effluent within regulatory limits.
 26. A process forincinerating combustible material including the steps of: deliveringcombustible material and inlet gases to a primary combustion chamber,the inlet gases having an oxygen content of at least 50 vol. %; burningthe combustible material with the oxygen of the inlet gases in theprimary combustion chamber producing flue gases and solid particulatesas thermal decomposition products of the burnt combustible material;passing the flue gases and particulates to a secondary combustionchamber where the combustion occurs at a temperature from about 850° C.to about 1500° C.; cooling the flue gases exiting the secondarycombustion chamber; returning a portion of the cooled flue gases to atleast one of the combustion chambers where the cooled gases moderate thetemperature in the at least one chamber; and passing the remainingportion of cooled flue gases on to a flue gas purification system wherepollutants in the flue gases and particulates are substantiallyconverted to benign compounds or removed entirely before the flue gasesare emitted into the atmosphere.
 27. A process according to claim 26,where the combustion in the secondary combustion chamber in said step ofpassing the flue gases is effected at a temperature from about 950° C.to about 1350° C.
 28. A process according to claim 27, where thecombustion in the secondary combustion chamber in said step of passingthe flue gases is effected at a temperature from about 1050° C. to about1200° C.
 29. A system for incinerating waste, the system including: aprimary combustion chamber in which waste, delivered from a source ofwaste, is burned to produce flue gases and solid particulates; first andsecond gas inlets for providing inlet gas streams into said primarycombustion chamber, both of said inlets providing inlet gas streamshaving an oxygen content of at least 50 vol. %, where said first gasinlet provides inlet gases adjacent to the waste in said chamber andwhere said second gas inlet provides inlet gases above the flames of theburning waste; a secondary combustion chamber in flow communication withsaid primary combustion chamber for burning non-combusted or partiallycombusted flue gases and solid particulates arriving from said primarycombustion chamber; a heat exchanger in flow communication with saidsecondary combustion chamber for cooling flue gases arriving from saidsecondary chamber; a particulate separator for separating solidparticulates from the flue gases arriving from said secondary combustionchamber; means for controlling the portion of the cooled flue gases tobe recirculated back to said at least one of said combustion chambers,said means for controlling in flow communication on the upstream sidewith said heat exchanger and in flow communication on the downstreamside with at least one of said combustion chambers, said recirculatedcooled flue gases cooling the temperatures in at least one of saidchambers; and a flue gas purification system in communication with saidmeans for controlling the portion of the cooled flue gases, saidpurification system purifying the flue gases not recirculated back intoat least one of said combustion chambers.
 30. A system according toclaim 29, wherein said means for controlling is valves.
 31. A systemaccording to claim 30 further including a control system to monitor aparameter in at least one combustion system and accordingly activatingsaid valves so as to control the portion of cooled flue gases returnedto at least one of said combustion chambers.
 32. A system according toclaim 31 wherein said at least one of said chambers is said primarycombustion chamber.
 33. A system according to claim 31 wherein said atleast one of said chambers is said secondary combustion chamber.
 34. Asystem according to claim 29 wherein said primary and secondary chambersare part of a single unit, said chambers positioned distant from oneanother so that their operation and control are independent of eachother.
 35. A process for incinerating combustible material including thesteps of: delivering combustible material and inlet gases to a primarycombustion chamber, the inlet gases having oxygen content of at least 50vol. %; burning the combustible material with the oxygen of the inletgases in the primary combustion chamber producing flue gases and solidparticulates as thermal decomposition products of the burnt combustiblematerial; passing the flue gases and particulates to a secondarycombustion chamber where further combustion occurs; cooling the fluegases exiting the secondary combustion chamber; returning a portion ofthe cooled flue gases to the secondary combustion chamber where thecooled gases moderate the temperature in the secondary combustionchamber; and passing the remaining portion of cooled flue gases on to aflue gas purification system where pollutants in the flue gases andparticulates are substantially converted to benign compounds or removedentirely before the flue gases are emitted into the atmosphere.